1. Field of the Invention
The present invention relates to a control device for an internal combustion engine that has a fuel reformer.
2. Description of Related Art
A low octane fuel is highly ignitable and, therefore, appropriate for running an engine under conditions for lean burning or for igniting a compressed homogenous fuel mixture, both of which are effective in improving fuel economy. However, the low octane fuel is more likely to cause knocking during high load running and, accordingly, is not appropriate for high load running. In contrast to the low octane fuel, a high octane fuel is less likely to cause knocking and, therefore, appropriate for high load running. However, the high octane fuel is not highly ignitable and is not appropriate either for running the engine under conditions for lean burning or the like. A vehicle that can use fuels of different octane ratings depending on an operating range may have both improved engine output and greater fuel economy. In view of these advantages, a technology has been proposed in which a fuel externally supplied to a vehicle is reformed to a fuel that has a different octane rating, and the fuel with a desired octane rating that matches running conditions of the engine is supplied to the engine.
However, such a conventional technology does not take account of the quantity of remaining reformed fuel. For example, if there is insufficient low octane fuel, a high octane fuel must be added to offset the insufficiency. This increases the octane rating and degrades ignitability in low load burning conditions, with the result that the engine is more likely to produce unstable burning. On the other hand, if there is insufficient high octane fuel, a low octane fuel must be added to offset the insufficiency. This decreases the octane rating and makes the engine more likely to fail during high load burning conditions due to knocking.